1. Field of the Invention
The present invention relates to a semiconductor device having a circuit including a thin film transistor (hereinafter, TFT) and a manufacturing method thereof. For example, the present invention relates to an electronic device having, as a component part, an electro-optic device typified by a liquid crystal display panel or a light-emitting display device with an organic light-emitting element.
Note that a semiconductor device in this specification means a generic device which can operate with use of semi-conductive characteristics and includes all types devices such as an electro-optic device, a semiconductor circuit and an electronic device.
2. Description of the Related Art
In recent years, a technology for forming a thin film transistor (TFT) using a semiconductor thin film (a thickness of around several nanometers to several hundred nanometers) formed over a substrate having an insulating surface has been attracting attention. Thin film transistors are broadly applied to electronic devices such as an IC or an electro-optic device, and are particularly developed as switching elements for image display devices at a rapid rate.
Conventionally, in an active matrix type liquid crystal display device with TFT driving, a plurality of scanning lines and data lines are arranged vertically and horizontally on a substrate and a large number of TFTs are provided at intersections of the lines. A gate wiring, a source electrode and a drain electrode of each TFT are electrically connected to the scanning line, the data line and a pixel electrode, respectively.
In a transmissive liquid crystal display device, a transparent conductive film typified by ITO is used as the pixel electrode. The pixel electrode formed of a transparent conductive film is provided over an interlayer insulating film for insulating it from wirings such as a data line or a scanning line. Therefore, the pixel electrode is electrically connected to the drain electrode through a contact hole formed in the interlayer insulating film.
Specifically, there is a problem of a signal delay due to wiring resistance in manufacturing a display device for large-size display. Therefore, a material having low electric resistance, typically aluminum, is used as the material for wirings or electrodes.
Aluminum used as a material of wirings or electrodes is incompatible with ITO used as a material of a pixel electrode in the junction, and thus there is a problem in that an electric corrosion is caused when the both are directly contacted.
When an aluminum film and an ITO film are directly contacted, aluminum is oxidized and ITO is reduced at the contact surface, and thus contact resistance is varied. It is known that this is a phenomenon of an electrochemical reaction caused due to differences in the electrode potentials of aluminum and ITO.
There is proposed a technique of preventing an electric corrosion with ITO by providing a high-melting point metal film (such as a titanium film), a high-melting metal compound film (such as a titanium nitride film) or the like between an aluminum wiring (or electrode) and the ITO, when connecting wirings or electrodes formed using the both films, which are incompatible with each other.
References 1, 2, and 3 describe that the connection between a drain of a thin film transistor and ITO as a pixel electrode is made by stacking a titanium film, an aluminum film, and a titanium film (Reference 1: Japanese Patent Laid-Open No. H9-45927, Reference 2: Japanese Patent Laid-Open No. H10-32202, and Reference 3: Japanese Patent Laid-Open No. H6-232129).
Reference 4 describes that the connection between a drain of a thin film transistor and ITO as a pixel electrode is made by stacking a titanium film and an aluminum film, and Reference 5 describes that the connection between a drain of a thin film transistor and ITO as a pixel electrode is made by stacking a titanium nitride film and an aluminum film (Reference 4: Japanese Patent Laid-Open No. 2004-6974 and Reference 5: Japanese Patent Laid-Open No. H8-330600).
However, when a titanium film or a titanium nitride film is formed between an aluminum wiring (or electrode) and ITO, wiring resistance is increased and power consumption is also increased if the size of a display screen is made larger. The wiring resistance can be decreased by enlarging the cross sectional area of a metal film to serve as a wiring; however, if the film thickness is made thicker and the cross sectional area is increased, a step between a surface of a substrate and a surface of a wiring having the thick film thickness is produced, which is a cause of an orientation failure of a liquid crystal.
In an active matrix type light-emitting device with TFT driving, there is a case where a transparent conductive film is used as an anode (or cathode) of a light-emitting element. Similarly, the anode formed of a transparent conductive film is formed over an interlayer insulating film for insulating it from various types of wirings. Therefore, the above described electric corrosion is similarly generated when ITO is used as an anode and connected to an electrode (aluminum) of a TFT.